COMPILER CONSTRUCTION Principles and Practice Kenneth C. Louden

8. Code Generation

8.1 Intermediate Code and Data Structures for Code Generation

8.1.1 Three-Address Code

8.1.2 Data Structures for the Implementation of Three-Address Code

8.1.3 P-Code

8.2 Basic Code Generation Techniques

8.2.1 Intermediate Code or Target Code as a Synthesized Attribute

8.2.2 Practical Code Generation

8.2.3 Generation of Target Code from Intermediate Code

Code generation from intermediate code involves either or both of two standard techniques: Macro expansion and Static simulation Macro expansion involves replacing each kind of intermediate code instruction with an equivalent sequence of target code instructions Static simulation involves a straight-line simulation of the effects of the intermediate code and generating target code to match these effects

Consider the expression (x=x+3) +4, translate the P-code into three-address code: Lad x Lod x Ldc 3 Adi t1=x+3 Stn x=t1 Ldc 4 Adi t2=t1+4 We perform a static simulation of the P-machine stack to find three-address equivalence for the given code

Now consider the case of translating from three-address code to P-code, by simple macro expansion. A three-address instruction: a = b + c Can always be translated into the P-code sequence lda a lod b lod c adi sto

Then, the three-address code for the expression (x=x+3)+4: T1 = x + 3 X = t1 T2 = t1 + 4 Can be translated into the following P-code: Lda t1 Lod x Ldc 3 Adi Sto Lad x Lod t1 Lda t2 Ldc 4

Contents Part Two 8.3 Code Generation of Data Structure Reference Part One 8.1 Intermediate Code and Data Structure for code Generation 8.2 Basic Code Generation Techniques Part Two 8.3 Code Generation of Data Structure Reference 8.4 Code Generation of Control Statements and Logical Expression 8.5 Code Generation of Procedure and Function calls Other Parts 8.6 Code Generation on Commercial Compilers: Two Case Studies 8.7 TM: A Simple Target Machine 8.8 A Code Generator for the TINY Language 8.9 A Survey of Code Optimization Techniques 8.10 Simple Optimizations for TINY Code Generator

8.3 Code Generation of Data Structure References

8.3.1 Address Calculations

(1) Three-Address Code for Address Calculations The usual arithmetic operations can be used to compute addresses Suppose wished to store the constant value 2 at the address of the variable x plus 10 bytes t1 = &x +10 *t1 = 2 The implementation of these new addressing modes requires that the data structure for three-address code contain a new field or fields For example, the quadruple data structure of Figure 8.4 (page 403) can be augmented by an enumerated address-mode field with possible values none, address, and indirect

8.3.2 Array References

The offset is computed from the subscript value as follows: First, an adjustment must be made to the subscript value if the subscript range does not begin at 0 Second, the adjusted subscript value must be multiplied by a scale factor that is equal to the size of each array element in memory Finally, the resulting scaled subscript is added to the base address to get the final address of the array element. The address of an array element a[t] : b a s e _ a d d ress (a) + (t - lower_bound (a)) * element_size (a)

(1) Three-Address Code for Array References Introduce two new operations： One that fetches the value of an array element t2= a[t1] And one that assigns to the address of an array element a[t2]= t1 For an example: a[i+1] = a [j*2]+3 Translate into the three-address instructions ( with the symbols: =[], []=) t1 = j * 2 t2 = a [t1] t3 = t2 + 3 t4 = i + 1 a [t4] = t3

The above example can be finally translated into: Writing out the addresses computations of an array element directly in the code, The above example can be finally translated into: t1 = j * 2 t2 = t1 * elem_size(a) t3 = &a + t2 t4 = *t3 t5 = t4 + 3 t6 = i + 1 t7 = t6 * elem_size (a) t8 = &a + t7 *t8 = t5

(2) P-Code for Array References Use the new address instructions ind and ixa. The above example a[i+1] = a [j*2]+3 Will finally become: lda a lod i ldc 1 a d i ixa elem_size(a) lod j ldc 2 m p i ind 0 ldc 3 a d I sto

Array reference generated by a code generation procedure. ( a [ i + 1 ] = 2 ) + a [ j ] lda a lod i ldc 1 a d i ixa elem_size(a) ldc 2 s t n lod j ind 0 adi

The code generation procedure for p-code: Void gencode( syntaxtree t, int isaddr) {char codestr[CODESIZE]; /*CODESIZE = max length of 1 line of p-code */ if (t != NULL) { switch(t->kind) { case OpKind: switch (t->op) { case Plus: if (is Addr) emitcode(“Error”); else { genCode(t->lchild, FALSE); genCode(t->rchild, FALSE); emitcode(“adi”);} break;

case Assign: genCode(t->lchild, TRUE); genCode(t->rchild, FALSE); emitcode(“stn”);} break; case Subs: sprintf(codestr,”%s %s”,”lda”, t->strval); emitcode(codestr); gencode(t->lchild,FALSE); sprintf(codestr,”%s%s%s”, “ixa elem_size(“,t->strval,”)”); if (!isAddr) emitcode (“ind 0”);

default: emitcode(“Error”); break; } case ConstKind: if (isAddr) emitcode(“Error”); else { sprintf(codestr,”%s %s”, ”ldc”,t->strval); emitCode(codestr);

case IdKind: if (isAddr) sprintf(codestr,”%s %s”,”lda”,t->strval); else sprintf(codestr,”%s %s”,”lod”,t->strval); emitcode(codestr); break; default: emitCode(“Error”); }

(4) Multidimensional Arrays For an example, in C an array of two dimensions can be declared as: Int a[15][10] Partially subscripted, yielding an array of fewer dimensions: a[i] Fully subscripted, yielding a value of the element type of the array: a[i][j] The address computation can be implemented by recursively applying the above techniques

8.3.3 Record Structure and Pointer References

For example, the C declarations: Computing the address of a record or structure field presents a similar problem to that of computing a subscripted array address First, the base address of the structure variable is computed; Then, the (usually fixed) offset of the named field is found, and the two are added to get the resulting address For example, the C declarations: Typedef struct rec { int i; char c; int j; } Rec; … Rec x;

(Other memory) x.j x.c x.i Offset of x.j Memory allocated to x Offset of x.c Base address of x

1) Three-Address Code for Structure and Pointer References Use the three-address instruction t1 = &x + field_offset (x,j) x.j = x.i; be translated into t2 = &x + field_offset (x,i) *t1 = *t2 Consider the following example of a tree data structure and variable declaration in C: typedef struct treeNode { int val; struct treeNode * lchild, * rchild; } TreeNode;

translate into the three-address code typedef struct treeNode { int val; struct treeNode * lchild, * rchild; } TreeNode; . . . TreeNode *p; p -> lchild = p; p = p -> rchild; translate into the three-address code t1 = p + field_offset ( *p, lchild ) *t1 = p t2 = p + field_offset ( *p, rchild ) p = *t2

2) P-Code for Structure and Pointer References x.j = x.i translated into the P-code lda x lod field_offset (x,j) ixa 1 ind field_offset (x,i) sto

The assignments: p->lchild = p; p = p->rchild Can be translated into the following P-code. Lod p Lod field-offset(*p,lchild) Ixa 1 Sto Lda p Ind field_offset(*p,rchild) sto

8.4 Code Generation of Control Statements and Logical Expressions

If labels are to be eliminated in the generation of target code, The section will describe code generation for various forms of control statements. Chief among these are the structured if-statement and while-statement Intermediate code generation for control statements involves the generation of labels in manner, Which stand for addresses in the target code to which jumps are made If labels are to be eliminated in the generation of target code, The a problem arises in that jumps to code locations that are not yet known must be back-patched, or retroactively rewritten.

8.4.1 Code Generation for If – and While – Statements

Two forms of the if- and while-statements: if-stmt → i f ( e x p ) stmt | i f ( exp ) stmt e l s e stmt while-stmt → w h i l e ( e x p ) s t m t The chief problem is to translate the structured control features into an “unstructured” equivalent involving jumps Which can be directly implemented. Compilers arrange to generate code for such statements in a standard order that allows the efficient use of a subset of the possible jumps that target architecture might permit.

The typical code arrangement for an if-statement is shown as follows:

While the typical code arrangement for a while-statement

Three-Address Code for Control Statement For the statement: if ( E ) S1 e l s e S2 The following code pattern is generated: <code to evaluate E to t1> if_false t1 goto L1 <code for S1> goto L2 label L1 <code for S 2> label L2

Three-Address Code for Control Statement Similarly, a while-statement of the form while ( E ) S Would cause the following three-address code pattern to be generated: label L1 <code to evaluate E to t1> if_false t1 goto L2 <code for S> goto L1 label L2

P-Code for Control Statement For the statement if ( E ) S1 else S 2 The following P-code pattern is generated: <code to evaluate E> fjp L1 <code for S 1> ujp L2 lab L1 <code for S 2> lab L2

P-Code for Control Statement And for the statement while ( E ) S The following P-code pattern is generated: lab L1 <code to evaluate E> fjp L2 <code for S> ujp L1 lab L2

8.4.2 Generation of Labels and Back-patching

One feature of code generation for control statements that can cause problems during target code generation is the fact that, in some cases, jumps to a label must be generated prior to the definition of the label itself A standard method for generating such forward jumps is either to leave a gap in the code where the jump is to occur or to generate a dummy jump instruction to a fake location Then, when the actual jump location becomes known, this location is used to fix up, or back-patch, the missing code

During the back-patching process a further problem may arise in that many architectures have two varieties of jumps, a short jump or branch ( within 128 bytes if code) and a long jump that requires more code space In that case, a code generator may need to insert nop instructions when shortening jumps, or make several passes to condense the code

8.4.3 Code Generation of Logical Expressions

The standard way to do this is to represent the Boolean value false as 0 and true as 1. Then standard bitwise and and or operators can be used to compute the value of a Boolean expression on most architectures A further use of jumps is necessary if the logical operations are short circuit. For instance, it is common to write in C: if ((p!=NULL) && ( p->val==0) ) ... Where evaluation of p->val when p is null could cause a memory fault Short-circuit Boolean operators are similar to if-statements, except that they return values, and often they are defined using if-expressions as a and b :: if a then b else false and a or b :: if a then true else b

To generate code that ensures that the second sub-expression will be evaluated only when necessary Use jumps in exactly the same way as in the code for if-statements For instance, short-circuit P-code for the C expression ( x ! = 0 ) & & ( y = = x ) is: lod x ldc 0 n e q fjp L1 lod y e q u ujp L2 lab L1 lod FALSE lab L2

8.4.4 A Sample code Generation Procedure for If- and While- Statements

Exhibiting a code generation procedure for control statements using the following simplified grammar: stmt → if-stmt | while-stmt | b r e a k | o t h e r if-stmt → i f ( exp ) stmt | i f ( e x p ) stmt e l s e s t m t while-stmt → w h i l e ( e x p ) s t m t exp → t r u e | f a l s e

The following C declaration can be used to implement an abstract syntax tree for this grammar: typedef enum { ExpKind, IfKind, WhileKind, BreakKind, OtherKind } NodeKind; typedef struct streenode { NodeKind kind; struct streenode * child[3] ; int val; /* used with ExpKind */ } STreeNode; typedef STreeNode * SyntaxTree;

Using the given typedef’s and the corresponding syntax tree structure, a code generation procedure that generates P-code is given as follows: Void genCode(SyntaxTree t, char* lable) { char codestr[CODESIZES]; char *lab1, *lab2; if (t!=NULL) switch (t->kind) {case ExpKind: if (t->val==0) emitCode(“ldc false”); else emitcode(“ldc true”); break;

genCode(t->child[0], label); lab1 = genLable(); case IfKind: genCode(t->child[0], label); lab1 = genLable(); sprintf(codestr,”%s %s”, “fjp”,lab1); emitcode(codestr); gencode(t->child[1],label); if (t->child[2]!=NULL) { lab2=genlable(); sprintf(codestr,”%s %s”,”ujp”,lab2); emitcode(codestr);} sprintf(codestr,”%s %s”,”lab”,lab1); { gencode(t->child[2],lable); sprintf(codestr,”%s %s”,”lab”,lab2); break;

case WhileKind; lab1=genlab(); sprintf(codestr,”%s %s”, “lab”,lab1); emitcode(codestr); gencode(t->child[0],label); lab2=genlabel(); sprintf(codestr,”%s %s”, “fjp”,lab2); gencode(t->child[1],lab2); sprintf(codestr,”%s %s”, “ujp”,lab1); sprintf(codestr,”%s %s”, “lab”,lab2); break;

case BreakKind: sprintf(codestr,”%s %s”, “ujp”,label); emitcode(codestr); break; case OtherKind: emitcode(“other”); Default: }

The above procedure generates the code sequence For the statement, if (true) while (true) if (false) break else other The above procedure generates the code sequence ldc true fjp L1 lab L2 fjp L3 ldc false fjp L4 ujp L3 ujp L5 lab L4 Other lab L5 ujp L2 lab L3 Lab L1

8.5 Code Generation of Procedure and Function Calls

8.5.1 Intermediate Code for Procedures and Functions

First, there are actually two mechanisms that need descriptions: The requirements for intermediate code representations of function calls may be described in general terms as follows First, there are actually two mechanisms that need descriptions: function/procedure definition and function/procedure call A definition creates a function name, parameters, and code, but the function does not execute at that point A call creates values for the parameters and performs a jump to the code of the function, which then executes and returns

Intermediate code for a definition must include An instruction marking the beginning, or entry point, of the code for the function, And an instruction marking the ending, or return point, of the function Entry instruction <Code for the function body> Return instruction Similarly, a function call must have an instruction indicating the beginning of the computation of the arguments and an actual call instruction that indicates the point where the arguments have been constructed and the actual jump to the code of the function can take place Begin-argument-computation instruction <Code to compute the arguments > Call instruction

Three-Address Code for Procedures and Functions In three-address code, the entry instruction needs to give a name to the procedure entry point, similar to the label instruction; thus, it is a one-address instruction, which we will call simply entry. Similarly, we will call the return instruction return For example, consider the C function definition. int f ( int x, int y ) { return x + y + 1; } This will translate into the following three-address code: entry f t1 = x + y t2 = t1 + 1 return t2

Three-Address Code for Procedures and Functions For example, suppose the function f has been defined in C as in the previous example. Then, the call f ( 2+3, 4) Translates to the three-address code begin_args t1 = 2 + 3 arg t1 arg 4 call f

P-code for Procedures and functions The entry instruction in P-code is ent, and the return instruction is ret int f ( int x, int y ) { return x + y + 1; } Thus the definition of the C function f translates into the P-code ent f lod x lod y a d i ldc 1 r e t

P-code for Procedures and functions Our example of a call in C (the call f (2+3, 4) to the function f described previously) now translates into the following P-code: m s t ldc 2 ldc 3 a d i ldc 4 cup f

8.5.2 A Code Generation Procedure for Function Definition and Call

The grammar we will use is the following: program → decl-list exp decl-list → decl-list decl | ε decl → f n id ( param-list ) = e x p param-list → p a ram - list, id | id exp → exp + exp | call | num | id call → id ( arg-list ) arg-list → a rg-list, exp | exp An example of a program as defined by this grammar is fn f(x)=2+x fn g(x,y)=f(x)+y g ( 3 , 4 )

We do so using the following C declarations: typedef enum {PrgK, FnK, ParamK, PlusK, CallK, ConstK, IdK} NodeKind ; typedef struct streenode { NodeKind kind; struct streenode *lchild,*rchild, * s i b l i n g ; char * name; /* used with FnK,ParamK,Callk,IdK */ int val; /* used with ConstK */ } StreeNode; typedef StreeNode * SyntaxTree;

Abstract syntax tree for the sample program : fn f(x)=2+x fn g(x,y)=f(x)+y g ( 3 , 4 )

Given this syntax tree structure, a code generation procedure that produces P-code is given in the following: Void genCode( syntaxtree t) { char codestr[CODESIZE]; SyntaxTree p; If (t!=NULL) Switch (t->kind) { case PrgK: p = t->lchild; while (p!=NULL) { gencode(p); p = p->slibing;} gencode(t->rchild); break;

case FnK: sprintf(codestr,”%s %s”,”ent”,t->name); emitcode(codestr); gencode(t->rchild); emitcode(“ret”); break; case ConstK: sprintf(codestr,”%s %d”,”ldc”,t->val); case PlusK: gencode(t->lchild); emitcode(“adi”); case IdK: sprintf(codestr,”%s %s”,”lod”,t->name);

case CallK: emitCode(“mst”); p = t->rchild; while (p!=NULL) {genCode(p); p = p->sibling;} sprintf(codestr,”%s %s”,”cup”,t->name); emitcode(codestr); break; default: emitcode(“Error”); }

Given the syntax tree in Figure 8 Given the syntax tree in Figure 8.13, the generated the code sequences: Ent f Ldc 2 Lod x Adi Ret Ent g Mst Cup f Lod y Ldc 3 Ldc 4 Cup g

End of Part Two THANKS